There are at present two subjects of vital importance in the legal framework of the animal production sector: the use of antibiotics that are growth promoters and the emission of residues to the environment, of the trace elements necessary both for promoting said growth and incorporated in feed.
Regarding growth-promoter antibiotics, these display great efficacy for improving production yields and preventing certain diseases, so that for more than 50 years they have made it possible to reduce production costs considerably. However, owing to the controversy concerning the possible development of resistance in certain strains of bacteria and its consequences for public health, in March 2002 the Committee of the European Union proposed a ban on these additives, which will be applied starting from 2005. Considerable repercussions are to be expected in the animal production sector, owing to the large increase in the costs of production.
In the case of trace elements, the considerable genetic improvement and physical development of production animals have led to an increase in demand for these nutrients to satisfy the requirements and ensure optimum development. In this sense, however, waste disposal is increasingly being regulated by legislation and the maximum permitted levels for inclusion in feed are steadily decreasing. Therefore recourse is being had increasingly to new sources of minerals (organic sources of minerals) with greater bioavailability and, accordingly, less likely to be eliminated in the feces. It should be pointed out that some inorganic sources of minerals, such as copper sulfate and zinc oxide, when administered at high doses (250 ppm and 1500-3000 ppm respectively) produce a considerable growth-promoter effect, mainly through their bactericidal action in the intestine, but said doses are far higher than those laid down by the environmental legislation (175 and 250 ppm for copper and zinc, respectively), therefore we must also do without their benefits.
That is why in recent years the animal feed additives industry has devoted considerable effort to the development of new substances to replace growth-promoting antibiotics without posing a health risk, and to the search for organic sources of minerals that provide the levels required for optimum growth of the animal and greatly reduce the discharge of residues into the environment.
Organic acids have proved very effective as intestinal sanitizing agents and improvers of production parameters in livestock and they therefore represent one of the most suitable alternatives to growth-promoting antibiotics. Among them, formic acid and butyric acid can be regarded as the most effective in monogastric animals owing to their recognized bactericidal effect and growth stimulation of the intestinal villi, which improve intestinal integrity and increase the absorption of nutrients. Supplements of iron (Fe) in the diet of livestock, by means of formate (WO 99/62355), or supplements of chromium (Cr+6) or manganese (Mn+7), by means of propionates (WO 98/33398), are known in this context.
The organic sources of minerals available as supplements for animal nutrition comprise:                Metal chelates with amino acids: molar ratio from 1:1 to 1:3.        Metal/amino acid complexes: formed by covalent bonding of an (unspecified) amino acid and a metal.        Complexes of specific amino acids with a metal: constituted of a specific amino acid and a metal.        Proteinates: resulting from the chelation of a hydrolyzed protein with a metal.        Polysaccharide/metal complexes.        Metal carboxylates: salts of various carboxylic acids with divalent metals. Used for the most part as organic mineral supplements, with greater bioavailability than the inorganic sources.        
Against this background, one of the objects of the present invention relates to the production of combined molecules of organic acids of recognized efficacy in animal production, concretely formic and butyric acids, and inorganic salts of zinc and copper. This combination displays a synergistic effect which boosts the effectiveness of both substances in improving the production parameters and increases the bioavailability of the metals, permitting the use of copper and zinc as promoter substances, but keeping their level of inclusion in the feed within the established legal limits.
Another object of the present invention is the production of derivatives of the aforementioned metal carboxylates which are carboxylate-aminoates of divalent metals or carboxylate-methioninate hydroxy analogs of divalent metals. This combination displays an even greater synergistic effect which boosts the effectiveness of these substances in improving the production parameters and increases the bioavailability of the metals, further facilitating the use of divalent metals as promoters, but keeping their level of inclusion in the feed within the established legal limits.
Another object of the present invention is to develop a method of production, both of metal carboxylates and of their metal carboxylate-aminoate or carboxylate-methioninate hydroxy analog derivatives, as an alternative to the conventional methods of synthesis in an aqueous medium that require the separation of the precipitated product from the solution and drying of said product.
A further object of the present invention is the use of the products obtained (metal carboxylates and their metal carboxylate-aminoate or metal carboxylate-methioninate hydroxy analog derivatives) as additives in the feed of monogastric production animals, with the aim of improving their productivity.
An advantage of the process described, relative to the conventional method in aqueous solution, is that it reduces the number of stages in the production process considerably, since operations such as product precipitation or filtration are avoided. Another advantage of this invention is that it provides a process for the production of carboxylates of divalent metals that is easy to implement on a large scale and at low cost since the process requires relatively low energy consumption. Furthermore, this method of production offers the additional advantage over the conventional method, that in some cases it increases the solubility with respect to some basic metal compounds. Yet another advantage of the invention is that an organic source of metal is obtained with a higher metal content.
Regarding its application, the compounds described in this specification have the advantage that their obvious growth-promoting effect in monogastric animals improves the production parameters, increasing the bioavailability of the metals and therefore reducing their emission to the environment.